Investment Securities Maturity Schedule (Details) - USD ($) $ in Thousands		Sep. 30, 2017	Dec. 31, 2016
Available-for-Sale, Amortized Cost
Due within one year		$ 83,120
Due after one year through five years		248,611
Due after five years through ten years		225,885
Due after ten years		367,884
Total before mortgage-backed securities		925,500
Total		1,866,151	$ 2,422,802
Available-for-Sale, Fair Value
Due within one year		83,270
Due after one year through five years		250,105
Due after five years through ten years		235,110
Due after ten years		376,791
Total before mortgage-backed securities		945,276
Total		1,886,517	2,425,477
Held-to-Maturity, Amortized Cost
Due within one year		0
Due after one year through five years		1,697
Due after five years through ten years		72,217
Due after ten years		581,214
Total before mortgage-backed securities		655,128
Amortized Cost		655,128	675,674
Held-to-Maturity, Fair Value
Due within one year		0
Due after one year through five years		1,756
Due after five years through ten years		74,597
Due after ten years		594,845
Total before mortgage-backed securities		671,198
Total		671,198	$ 689,089
Mortgage-backed securities
Available-for-Sale, Amortized Cost
Mortgage-backed securities	[1]	940,651
Available-for-Sale, Fair Value
Mortgage-backed securities	[1]	941,241
Held-to-Maturity, Amortized Cost
Mortgage-backed securities	[1]	0
Held-to-Maturity, Fair Value
Mortgage-backed securities	[1]	$ 0

[1] Mortgage-backed securities, which have prepayment provisions, are not assigned to maturity categories due to fluctuations in their prepayment speeds.